A number of significant applications require measurement of extremely low abundances of a particular isotope. For example, it is necessary to measure extremely low abundances of a particular isotope in archeology dating where radioactive species are used as chronometers. Often .sup.14 C is used for such dating. Measurement of low abundances is also used in geology and cosmology where .sup.10 Be is used as a tracer. Moreover, selected tracer isotopes are used in biological and industrial applications and in the detection of fission-product isotopes for environmental monitoring. Often such applications require instrument sensitivities which will allow measurements of ratios as small as 1 part in 10.sup.16. An instrument capable of measuring such an abundance ratio would have an abundance sensitivity which exceeds 10.sup.16. This abundance sensitivity is the quantity that defines a mass spectrometry's ability to measure a given abundance ratio of two neighboring isotopes of an element in the middle of the periodic table.
Three known techniques are presently used in measuring extremely low isotope abundance ratios. In one technique, a conventional mass spectrometer employs electric and magnetic fields to perform the mass selection. In another technique, high energy (MeV) mass spectrometry is used in conjunction with a tandem accelerator. Moreover, radioactive counting can be employed. Conventional mass spectrometers are not generally capable of abundance sensitivities beyond 10.sup.9, and frequently cannot achieve such sensitivities because of isobaric and molecular interferences. Certain high energy spectrometers are presently capable of abundance sensitivity measurements in the range of 10.sup.16 for certain light elements for example, .sup.14 C and .sup.36 Cl. These spectrometers prove useful for providing ultrasensitive measurements on a significant group of elements. However, the tandem accelerator-based spectrometers have several disadvantages. First, only elements having a negative ion bound state can be measured. Additionally, the ability to discriminate between isobars decreases markedly for elements having an atomic number greater than forty. This ability to discriminate between isobars is also ineffective for a certain class of elements regardless of the atomic number. Further, such spectrometers are costly to build, operate and maintain. Conventional radioactivity counting has a major draw back in that it can not be applied to the rare stable isotopes of interest and becomes impractical for small samples of long-lived nuclides.
Recently, a new technique of mass spectrometry commonly referred to as resonance ionization mass spectrometry or "RIMS", has been demonstrated successfully. Such technique is referred to in the International Journal of Mass Spectrometry Ion Physics, Volume 34, pages 89-97 (1980), in an article authored by D. W. Beekman, T. A. Callcott, S. D. Kramer, E. T. Arakawa, and G. S. Hurst. This technique employs resonantly enhanced laser multiphoton ionization as the ion source for a conventional mass spectrometer. For example, see U.S. Pat. No. 3,987,302 which is incorporated by reference herein together with the above identified article. As currently conceived, RIMS has the potential to eliminate isobaric and molecular interferences. The ultimate abundance sensitivity of a RIMS apparatus is limited by the sensitivity of the mass spectrometer which is 10.sup.9 for conventional mass spectrometers.
Accordingly, it is an object of the present invention to provide a method for ultrahigh abundance sensitivity measurement incorporating a mass spectro-meter. Such sensitivity measurements can be made on a large group of commercially and scientifically significant elements. Another object of the present invention is to provide a method for measuring isotope abundance ratios incorporating a step of preferentially ionizing a selected isotope by using Doppler-free resonant multiphoton ionization. It is also an object of the present invention to provide such an ultrahigh abundance sensitivity process capable of determining an abundance ratio of 1 part in 10.sup.12.